What if you have a lot of local modules? I wonder if you can do local builds against Dojo hosted on CDNs. Did you try it?

First, if you save off the prototype method and something monkey patches afterwards, you get the monkey patched version of the method which you might not be expecting.

Second, each time you call myAdder.add with your version, the interpreter has to look up the global Adder object, then it has to look up its prototype object, and then its add function (which is just an object), and finally it has to find and call its call function. This can be a lot of work for such a simple method depending on how many times the method will be called.

Third, with the introduction of a closure you use a bit more memory. Combined with my previous point, this will be something you have to decide: speed or memory use.

By the way, if you use your way you won’t need the function declaration and execution around your masking since you’re not saving anything off.

(function(){
myAdder.add = function(what){
return Adder.prototype.add.call(this, what) + 20;
};
})();
myAdder.add(5);

Seems to work without adding a new variable.